1. Field of the Invention
The present invention relates to an electron microscope having a spherical aberration corrector, such as a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM).
2. Description of Related Art
In a spherical aberration corrector for use in an electron microscope (such as a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM)), a so-called magnetic multipole element consisting of plural (four, eight, twelve, or the like) magnetic bodies around which coils are wound is used, the magnetic bodies being disposed on a plane perpendicular to the optical axis, as disclosed in Japanese Patent Laid-Open No. 2003-92078.
Spherical aberration corrector and spherical aberration correction systems described below pertain to a spherical aberration corrector that produces a hexapole field by means of a multipole element for a transmission electron microscope (TEM).
FIG. 3 is a diagram showing the configuration of a spherical aberration correction system 50 which uses a multipole element and is used in an electron microscope. As shown in this figure, a condenser lens 51, an aberration corrector 52, transfer optics 53, an objective lens 54, and a specimen 55 are placed in this order in the spherical aberration correction system 50. In the case of a transmission electron microscope (TEM), an image free of aberrations is formed from the specimen toward the aberration corrector. In the cases of scanning transmission electron microscope (STEM) and scanning electron microscope (SEM), an image of the light source is formed from the aberration corrector toward the specimen without producing aberrations.
The transfer optics built in the spherical aberration correction system of the electron microscope acts to make the working surface of the aberration corrector and the aberration introduction surface of the objective lens optically equivalent within the range of the primary orbit. Usually, the transfer optics is made up of two or one magnetic lenses having a principal plane. The transfer optics 53 shown in FIG. 3 consists of two lenses, i.e., first lens 53a and second lens 53b. The transfer optics made are not intended to show magnifying or demagnifying capabilities, which the lenses should intrinsically exhibit. The optics is designed such that the objective lens 54 and spherical aberration corrector 52 together focus light at a magnification of 1. Where the magnification is not 1, the magnification is close to 1.